The short answer is that "Source Magic" is Divinity's version of Blood Magic, or Black Magic, or Forbidden Magic. I.E. it's just another school of magic that's more powerful than regular magic in-lore, but pretty indistinguishable in-game. And it's called Source Magic (a very poor descriptor) primarily for the sourcery pun and Pratchett reference.
except not really... its not really forbidden or dark magic.. its first uses were for healing.. that said however the source was corrupted and is now more dark than it was.
